ARM: dts: dra7: add routable-irqs property for gic node

There is a IRQ crossbar device in the soc, which maps the
irq requests from the peripherals to the mpu interrupt
controller's inputs. The gic provides the support for such
IPs in the form of routable-irqs. So adding the property
here to gic node.

Signed-off-by: Sricharan R <r.sricharan@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Cc: Benoit Cousson <bcousson@baylibre.com>
Cc: Santosh Shilimkar <santosh.shilimkar@ti.com>
Cc: Rajendra Nayak <rnayak@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>